well you could do some of the work on your jem jr. yourself. Your main cost is going to be taking it to someone to do the work you want to do. The only thing that would require expensive tools you probably don't have is the fret level, and I am curious why you feel you need a fret level ? Do you have some dead spots on the neck ?
wiring in the pickups isn't very hard, and a soldering iron and solder isn't very much. You could always save the stock pickups and put them back in if you decide to sell it later on. You could also save the upgraded trem and put the stock back in for selling it.
those stock trems aren't the greatest but it sounds like yours needs to be set up correctly. The knife edges of the trem need to meet the trem posts at a 90 degree angle when the guitar is tuned the way you want it. If the trem is ****ed forward or back from the 90 degree point, you can get tuning instability. If the knife edges are worn or dirty, that can also impact your tuning stability.
anyways at most you'll sink $500, the more work you do yourself, the less you are paying someone else to do what you can do for free.
there are plenty of great used guitars out there which may suit what you are going for a cheaper price than what you are looking at spending on the jem jr.
the premium stuff isn't too bad. They don't have great resale value so you can usually pick them up reasonable. the 920s are great and can be had for less than 500.
I'm comparing it to my main guitar, which is a Warmoth tele I built and had fretwork done to, and has a very low and buzz free action. I've tried, but can't get the Jr's action anywhere near as low. Also, when I take a fret rocker to it, it pretty clearly has a lot of uneven frets.
I do have the trem level to the body, and last night I cleaned the knife edge and applied a little chapstick to it. Now, if I tune and then use the trem to slack the strings, it returns about 20 cents flat. Since I don't have any experience with Floyds, I guess I can't really say if this is unusual or not. Is this considered normal?
I could most likely upgrade the trem with a Gotoh myself, but I wouldn't be able to replace the studs. Would replacing the trem and leaving the studs be a significant upgrade?
The RG920 looks like it'd be a really good fit for me. If I can find one without the quilted maple top, I might go that route.